Charged particle beam apparatuses have many functions in a plurality of industrial fields, including, but not limited to, inspection of semiconductor devices during manufacturing, exposure systems for lithography, detecting devices and testing systems. Thus, there is a high demand for structuring and inspecting specimens within the micrometer and nanometer scale.
Micrometer and nanometer scale process control, inspection or structuring, is often done with charged particle beams, e.g. electron beams, which are generated and focused in charged particle beam devices, such as electron microscopes or electron beam pattern generators. Charged particle beams offer superior spatial resolution compared to, e.g. photon beams due to their short wavelengths.
In semiconductor technology not only miniaturization but also using the 3rd dimension takes place in order to get more and better functionality into electronic devices. In particular, using the 3rd dimension gives challenges to process control in which the surface structures need to be imaged with high resolution for metrology, inspection and defect review.
In the past it was mainly a resolution challenge since the surface structures were more or less planar or had reasonable aspect ratios. Going to 3-D devices, structures of several hundreds of nanometer have to be imaged containing aspect ratios of more than 20.
This change in device architecture provides the need for high resolution particle beam imaging systems, e.g. electron & ion beam systems, which are capable of imaging not only surfaces but also deep holes and side walls with good signal to noise ratio. Additionally the height information should be available and, if possible, measurable. Presently scanning electron microscope based tools are used for these purposes (CD-SEM, DR-SEM, EBI-tools). However, these tools reach their limits for the desired applications.
Normally in electron beam tools fine electron probes are generated. For example, a high brightness source (e.g. a thermal field emission or cold field emission source) generates an electron beam. The source (or virtual source) is imaged onto the sample surface. This is done by an objective lens and in many cases in combination with one or more condenser lenses. The condenser lens system can provide aperture angle adaption to achieve the optimum aperture angle in the optical system according to the used probe current. Additionally probe current adjustment and spot size variations can be performed with the condenser system.
The aperture angle itself is defined by a mechanical hole. The electron probe is scanned by a 1-, 2- or more stage deflection system across the sample for image generation. The generated signal particles, i.e. secondary electrons and/or backscattered electrons (SE and/or BSE) are detected by post lens, in-lens or pre-lens detection systems or combinations hereof.
Since normally low energy (<5 keV) electrons are used for the mentioned applications, advantageously retarding field optics are used which apply a high beam energy inside the column and which will generate the final landing energy next to the sample (inside the objective lens, between objective lens and sample or a combination hereof). The low landing energies mean that due to brightness limitations to obtain a large current at the sample a large aperture angle must be used; this increases both spherical and chromatic aberrations.
For 3-D samples this kind of imaging has limitations as can be seen from FIG. 1. The triangles 11, 12, and 13 represent the beam with its divergence angle. This divergence angle is typically optimized and based on a compromise between diffraction, aberrations (spherical and/or chromatic) and e-e-interactions (electron-electron-interaction). Thus, the divergence angle is not freely or arbitrarily selectable. As previously mentioned the divergence angle should be as large as possible for obtaining high probe currents and sufficient signal to noise.
As can be seen for area 21 of specimen 9 no problems occur to image the area. In area 22 the lower edge can hardly be imaged, and imaging of the side wall is impossible. This is due to the divergence and the fact that the beam “touches” the protrusion as indicated by area 3. In area 23 both side walls and the lower corners cannot be imaged, as indicated by the second area 3. This is not only because of geometrical issues but also because of signal to noise issues. The signal from the bottom of the hole is low in comparison to the signal at the top. The signal electrons generated in area 3 for the triangle 13 illustrating the beam imaging area 23 are mixed with the signal electrons from the bottom of the hole and might even dominate the signal.
Part of the problem can be solved by tilting the beam during scanning the surface. Some hardware solutions for beam tilt and resulting benefits have been described. Yet, further benefits are desired for 3-D imaging on industrial standards.